I went off the power supply maximum output. 5.1 volts, 3 amps, so 15 watts per hour. 24hrs per day, 365 days a year, so 131,400 watt-hours, or 131kilowatt-hours. My electricity is about $0.25/kwh (advertised at 0.09/kwh, but when you add on bullshit fees, the final rate is much higher), so I came up with $32.85 as the maximum amount any device connected to that power supply could cost.

A realistic Pi 4B-only estimate is about A$8–A$12 per year in electricity

That's about what I calculated for my locale. Roughly $0.30–$0.85 per month, around $0.48/month at 4 W. Which is remarkable especially given what you can run on one.